Valve seat recession occurs in internal combustion engines when hot valves (most commonly the exhaust valves) contact the valve seat in the engine head. The heat from the valve creates a localized “weld” between the valve and the valve seat. When the valve is re-opened, material from the valve seat breaks away because the valve seat is made of softer material than the valve. Thus, the valve seat progressively recesses into the cylinder head.
The occurrence of valve seat recession is exacerbated when the engine is operating at high loads, such as when the vehicle is towing a load. Such high load conditions result in higher engine operating temperatures. For this reason, some fuels include lubricants to act as a cushion between the valve and the valve seat, which minimizes the direct contact between the components and minimizes valve seat recession.
Vehicles that have been modified to run on Liquefied Petroleum Gas (LPG) or Compressed Natural Gas (CNG) are particularly susceptible to valve seat recession because these fuels are dry burning fuels. In addition, these fuels combust at higher temperatures than petrol.
It is known to provide a lubrication system to lubricate the upper cylinder region of the engine and thus minimize the occurrence of valve seat recession. Such known systems deliver a lubricant to the engine inlet manifold downstream of the throttle body. This allows the lubricant to mix with air/fuel mixture prior to being drawn into the cylinders.
The known systems utilize inlet manifold vacuum pressure to draw lubricant from the reservoir into the manifold. This system requires the delivery rate to be set manually by an empirical flow rate method. Furthermore, it can be difficult to set the delivery rate of lubricant to provide the required amount of lubricant across all engine operating conditions.